Ok guys, I think it could be useful if we'll have an MP3 flash player on our forum like we have for youtube.
I can help you with it.Here is the player, it should be uploaded on the server.
After that go to admin cp->Custem BB-codes->add BB-code.
Title=MP3.
BB Code Tag Name=mp3.
Replace (big textarea)

I thought about it and what I think: we should use it not the way I given in my post. We have to use {option} for URL of sound & let users to put smth between [ mp3][/ mp3] as a caption (just place {param} there).

__________________What about a 500+ symbols long, colored signature with URL allowed and size limited to 7?